Voltage-controlled oscillators (VCO) are commonly used in a phase-locked loop (PLL), a reference clock, a frequency synthesizer, etc., in both wireless and wire line communication systems. Phase noise is a parameter of the VCO indicating the quality of the VCO signals. The total phase noise in a VCO includes the noise generated by the transistors in the vicinity of the flicker frequency and the integer multiples of the oscillation frequency of the VCO. The oscillation frequency is commonly called the fundamental frequency or the resonant frequency. The value 1/f is used to refer to the flicker frequency where f is the frequency of the noise. Generally, the flicker noise dominates the 1/f3 shaped part of the noise spectrum, while the thermal noise dominates the 1/f2 shaped part of the noise spectrum.
As complementary metal oxide semiconductor (CMOS) technology downscales, the 1/f flicker frequency of transistors in the VCO tends to increase due to the smaller size of the transistors. As a result, the inductor quality factor referred to as the value Q and the phase noise of the CMOS VCO become worse due to the small distance between the metal and the substrate of the transistors.
In an approach, an inductor capacitor (LC) resonator circuit is added as a load in the LC tank of the VCO to affect the frequency tuning range and the inductor value Q of the VCO. The phase noise contributed by the flicker noise, however, is not suppressed effectively.
In another approach, the VCO is configured such that the LC resonator circuit functions as an open circuit at the oscillation frequency and is electrically shorted at the 2nd harmonic frequency. In this approach, the phase noise generated from the noise down-conversion around the 2nd harmonic frequency is suppressed by the circuit that is electrically shorted at the 2nd harmonic frequency. The phase noise contributed by the flicker noise, again, is not suppressed effectively.
Like reference symbols in the various drawings indicate like elements.